In a mass spectrometer, an ion transport optical system, which is called an ion lens or ion guide, is used to converge ions sent from the previous stage, and in some cases accelerate them, in order to send them to a mass analyzer such as a quadrupole mass filter in the subsequent stage. One type of such ion transport optical system conventionally used is a multipole rod type, such as a quadrupole or octapole system. In a quadrupole mass filter which is often used as a mass analyzer for separating ions in accordance with their mass-to-charge ratio, a pre-filter (which is also called pre-rods) composed of short quadrupole rod electrodes is provided in some cases in the previous stage of the main body of the quadrupole rod electrode in order to smoothly introduce ions into the main body. Such a pre-filter can also be regarded as one kind of an ion transport optical system.
FIG. 15(a) is a schematic perspective view of a general quadrupole rod type ion guide 710, and FIG. 15(b) is a plane view of the ion guide in a plane orthogonal to the ion optical axis C. The ion guide 710 is composed of mutually parallel four columnar (or tube-like) rod electrodes 711 through 714 which are arranged in such a manner as to surround the ion optical path C. Generally, as illustrated in FIG. 15(b), the same radio-frequency voltage V·cos ωt is applied to two rod electrodes 711 and 713 facing across the ion optical axis C, and a radio-frequency voltage −V·cos ωt which has the same amplitude and reversed phase as the aforementioned radio-frequency voltage V·cos ωt is applied to two rod electrodes 712 and 714 which are placed next to the rod electrodes 711 and 713 in the circumferential direction. The radio-frequency voltages ±V·cos ωt applied as just described form a quadrupole radio-frequency electric field in the space surrounded by the four rod electrodes 711 through 714. In this electric field, ions can be converged into the vicinity of the ion optical axis C and transported into the subsequent stage, while being oscillated.
FIG. 16 is a plane view of an octapole rod type ion guide 720 in a plane orthogonal to the ion optical axis C. In the octapole rod type, eight columnar or tube-like rod electrodes 721 through 728 are arranged at the same angular intervals around the ion optical axis C as if they touch an inscribed circle. The radio-frequency voltages applied to each of the rod electrodes 721 through 728 in this case are also the same as in the case of the quadrupole.
In a quadrupole or multipole (more than four) rod type ion transport optical system as previously described, the shape of the radio-frequency electric field formed in the space surrounded by the rod electrodes differs in accordance with the number of their polar elements. This difference is also accompanied by a change in the ion optical properties such as an ion beam convergence, ion transmission, ion acceptance, and mass selectivity. Generally, a quadrupole which has a small number of poles shows a preferable beam convergence and mass selectivity by a collisional cooling with a neutral molecule; increasing the number of poles deteriorates the beam convergence and mass selectivity deteriorate while improving the ion transmission and ion acceptance.
As just described, in a conventional type ion transport optical system, the ion optical properties differ corresponding to the number of poles. Therefore, the ion transport optical system is generally designed in such a manner that the appropriate number of poles is selected in accordance with the relationship between the atmosphere (e.g. gas pressure) in which it is used and the ion optical elements provided in the previous stage and subsequent stage, and that parameters such as the rod electrode's radius and length are determined under the condition of the number of poles. However, the conventional type ion transport optical system has a disadvantage in that the flexibility of the selection of parameters is little and therefore an ion transport optical system having optimal ion optical properties suitable for the purpose cannot be always used, which may lead to the difficulty in increasing the detection sensitivity and accuracy.
In recent years, a higher sensitivity, higher accuracy, higher throughput, and other better properties in a mass spectrometer have been required in order to deal with the growing diversity and complexity of the kind of substances to be analyzed, the demand for a prompt analysis, and other requests. In order to meet such demands, improvement of the performance is required also for an ion transport optical system. However, in practice, the performance improvement based on a conventional multipole rod type configuration has limitations for the aforementioned reasons.    [Patent Document 1] Japanese Unexamined Patent Application Publication No. 2000-149865    [Patent Document 2] Japanese Unexamined Patent Application Publication No. 2001-351563